Method and system for a unicast endpoint client to access a multicast internet protocol (IP) session

ABSTRACT

Unicast endpoint clients ( 110, 111, 115 ) on an IP Unicast network ( 107, 108 ) are provided access to Multicast sessions on an IP Multicast network ( 101 ) through a Multicast-Unicast gateway server ( 120, 121 ). The server obtains information about sessions on the Multicast network and makes such information available to a Unicast client on the Unicast network upon request by the client. Upon being presented with a list describing the subject matter of each session, the user at the Unicast client selects the session to which he or she wants to join, which causes the Multicast-Unicast server to join the appropriate session on behalf of the requesting client for each media type in which the joining client wants to be a participant. The server then sets a bidirectional Unicast User Datagram Protocol (UDP) stream between itself and the client. All packets then received by the server from the Unicast client are address-translated to the appropriate Multicast session address. In addition, all packets received by the server on the Multicast session address are address-translated and sent to the Unicast client. The Unicast client is then able to participate in the Multicast session as both a sender and a receiver of packets to and from other Unicast and Multicast clients which are active during the session. Further, the Unicast client is capable of creating a new session, recording a session in the network for later retrieval and playback, and creating and accessing low bandwidth versions of existing sessions.

TECHNICAL FIELD

This invention relates to data communications, and more particularly, toproviding a Unicast endpoint client on a Unicast network with theability to access a Multicast session on an Multicast network.

BACKGROUND OF THE INVENTION

In conventional packet, frame or cell based systems there are typicallytwo modes of communication: point-to-point (also known as Unicast) andpoint-to-multipoint (also known as Multicast). Multicast addressestypically differ from Unicast addresses in that they refer to anintermediate abstraction known as a group. All senders address theirtransmitted information to this group and all receivers are “tuned” to“listen” to that address to receive the information transmitted to thatgroup by the senders. The senders of information are thus effectivelyde-coupled from the set of receivers. Senders do not need to know whothe receivers are—they simply transmit packets addressed to the group.Similarly, receivers do not need to know who the senders are—they simplysend a request to the network (routers) to join a specific group ofinterest.

Multimedia distribution and conferencing/collaboration systems areadvantageously and efficiently supported by Multicast communicationmethods. As will be used herein, a specific Multicast communication isreferred to as a session. In the prior art, it is not possible forUnicast endpoints to access Multicast sessions, due to the differencesin addressing modes and receiving modes. This disadvantageously limitsthe ability of a user at a Unicast endpoint client to participate insessions in which they have interest and in which they could be anactive participant.

SUMMARY OF THE INVENTION

In accordance with the present invention, Unicast endpoint clients areenabled to access Multicast sessions. Furthermore, in accordance withthe present inventions, Unicast endpoint clients are enabled to requestnetwork-based recording systems to perform recordings of the Multicastsession on behalf of clients. Even furthermore, in addition to LANattached endpoints, analog dial-up (e.g., 28.8 kbps) as well as ISDNUnicast endpoints are enabled to access appropriately bandwidth-reducedversions of nominally high bandwidth sessions.

Inter-connectivity between a Unicast client connected to a Unicastnetwork and one or more Multicast clients connected to a Multicastnetwork is effected through a Multicast-Unicast Server (MUS), inaccordance with the present invention. Such a server obtains informationabout sessions on the Multicast network and makes such informationavailable to the Unicast client on the Unicast network upon request bythe client. Upon being presented with a list describing the subjectmatter of each session, the user at the Unicast client selects thesession to which he or she wants to join, which causes theMulticast-Unicast server to join the appropriate session on behalf ofthe requesting client for each media type for which the joining clientwants to be a participant. The server then sets a bi-directional UnicastUser Datagram Protocol (UDP) stream between itself and the client. Allpackets then received by the server from the Unicast client areaddress-translated to the appropriate Multicast session address. Inaddition, all packets received by the server on the Multicast sessionaddress are address-translated and sent to the Unicast client. TheUnicast client is then able to participate in the Multicast session asboth a sender and a receiver of packets to and from other Unicast andMulticast clients which are active during the session. Further, such aUnicast client, if authenticated to do so, is capable of creating a newsession, and of recording a session in the network for later retrievaland playback. Furthermore, transcoding and rate adapting gateways aredeployed within the Multicast network that map existing sessions intolow bandwidth versions. If the Unicast client is a dial-up userconnected to the Unicast network over analog or ISDN facilities, theserate-controlled sessions can then be accessed by the dial-up users.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing Unicast clients connected on a UnicastInternet Protocol (IP) network accessing an IP Multicast network throughMulticast-Unicast Servers (MUS's);

FIG. 2 is a block diagram of a MUS;

FIG. 3 shows the software components of a client terminal that enablethe client to access a Multicast session;

FIG. 4 is a flowchart detailing the steps associated with a Unicastclient joining a Multicast session on the Multicast network;

FIG. 5 is a flowchart detailing the steps of a Unicast client creating aMulticast session;

FIG. 6 is a flowchart detailing the steps of a Unicast client recordinga Multicast session; and

FIGS. 7A and 7B together detail the steps of a Unicast clientrate-adapting/transcoding a Multicast session.

DETAILED DESCRIPTION

With reference to FIG. 1, an IP Multicast network 101 is shownincluding, as way of illustration, three interconnected MulticastRouters (MR) 102-1, 102-2, and 102-3. A well known currently in place IPMulticast network is the so-called MBONE (Multicast Backbone), which isa public shared IP Multicast network spanning many countries andcovering thousands of IP sub-networks. In addition to the MBONE,numerous private Intranets also use Multicast IP for intra-corporatecommunications. A plurality of multimedia client terminals are connectedto IP Multicast network 101, for example the MBONE. For way ofillustration only, in FIG. 1 two client terminals 103 and 104 are shownconnected to network 101 through respective Local Area Networks (LANs)105 and 106, respectively. Each LAN is connected through a customerpremises router (not shown) over, for example, a T1 line, Frame Relay(FR), ATM, or an X.25 connection. In accordance with Multicastcommunication, a sender of information of a particular media typetransmits packets of information to a particular address, which are thenautomatically distributed to each of the members of a group that haverequested to receive from that address. Advantageously, the networkperforms the replication functions necessary so that each receiverclient of the group can receive the packets transmitted to the addressby a sender or senders.

In accordance with the present invention, Unicast client terminalsconnected to a Unicast IP network are capable of joining in andparticipating with a Multicast session on an IP Multicast network. FIG.1 shows two Unicast IP networks 107 and 108. Illustrative Unicast clientterminals 110 and 111 are connected to network 107 through conventionalUnicast Routers (URs) 112 and 113, respectively. Unicast client terminal115 is connected to UR 116 within network 108. UR 116 in turn is shownconnected to another UR, UR 117 for example, which in turn is connectedto UR 118. Networks 107 and 108 are shown as being interconnectedthrough UR 112 and 118, thus enabling Unicast IP communication between aclient terminal connected to Unicast IP network 107and a client terminalconnected to Unicast IP network 108. The Unicast client terminals 110,111 and 115, for example, can be connected to networks 107 or 108 over aPlain Old Telephone Service (POTS) dial-up connection, an ISDNconnection or an Asynchronous Digital Subscriber Loop (ADSL) connection,each to a Local Exchange Carrier (LEC) (not shown) and from there to anInternet Service Provider (ISP) (not shown), which in turn is connectedto the Unicast IP network through a Unicast Router. Alternatively, aclient terminal can be connected to an ISP through a cable modem overcable facilities through a cable TV provider. Even further, the Unicastclient terminal could be connected to a LAN and to a customer premisesrouter to a UR over, for example, a Wide Area Network (WAN), T1facilities, Frame Relay, ATM, or X.25. In accordance with thisinvention, Unicast client terminals 110, 111, 115, for example, onUnicast IP networks 107 and 108, can participate in a Multicast IPsession on IP Multicast network 101. Specifically, such functionality isenabled through Multicast-Unicast Servers (MUS's), such as MUS 120 innetwork 107 and MUS 121 in network 108, which each interconnect theirrespective Unicast IP networks 107 and 108, with the IP MulticastNetwork 101.

The Multicast-Unicast Servers 120 and 121 function as gateways thatenable those Unicast-connected clients on their respective Unicast IPnetworks to access IP Multicast network 101. Specifically, each MUSthrough interaction with client software on the Unicast client on theUnicast IP network enables such client to join a group on the IPMulticast network by providing information relating to what sessions arein progress or scheduled on network 101. The MUS then receives and sendsdata on those groups within a session selected by client on behalf ofthe client. Thus, the MUS functions to convert the address of theMulticast IP packets of a requested-for group to the Unicast IP endpointaddress of the requesting client. Furthermore, the MUS functions totransmit packets it receives from the Unicast endpoint client to a listof any other Unicast endpoint receivers for the requested-for Multicastgroup on the same Unicast IP network or an interconnected Unicast IPnetwork (107 or 108), as well as to the Multicast group address itselfon the IP Multicast network 101 to enable the Multicast endpoints, 103and 104, for example, connected on that network to receive packetsoriginating from the Unicast client.

FIGS. 2 and 3 are block diagrams of an MUS 201 and a client terminal301, respectively, showing their functions that enable them to operatein accordance with the present invention. In FIG. 2 a SessionDescription Protocol (SDP) listener process 202 coupled to the IPMulticast network 101 listens for session announcements transmitted onMulticast network 101. Such directory announcements are repeatedly sentover the IP Multicast network 101. These “advertised” sessions arereceived by the SDP/SAP advertised process 203 and stored in a sessionsdatabase 204. Other sessions that are not announced and thus notreceived by the SDP listener process 202, are entered by a systemadministrator through a static sessions process 205 and also stored insessions database 204. An example of the latter might be a weatherchannel that is always present on a predetermined socket and is notrepeatedly announced over the IP Multicast network 101.

An HTTP server 206 can read the sessions database 204 so that when aclient connects to the server, the client is able to receive a listingof those Multicast sessions presently on the IP Multicast network 101.When the client connects to the server 206, the server executes a CGI(Common Gateway Interface) scripts 207 within the HTTP server 206 onbehalf of the client to present the information pertaining to thesessions to the client. Such information is presented back to the client301 (FIG. 3) through its web browser program 302 (in FIG. 3). The clientinitiates a request to join a session by selecting a URL on an HTML webpage presented to it by HTTP server 206. A message is thus sent from theclient 301 by web browser 302, which when received by server HTTP 206causes it to invoke certain actions. Specifically, HTTP server 206 sendsa response back to the client comprising a control message containinginformation indicating what tool needs to be used in order to join theselected session. Such tools are well known in the art and may include,for example, the Visual Audio Tool (VAT), the Visual Conference Tool(VIC), or the Internet Protocol Television Tool (IPTV). Furthermore, themessage indicates where the tools should be connected, i.e.,specifically, to which socket on the MUS the tool should be connected.On the client side, the client 301 then launches from launch program 303the specified tool from tool program 304 to the indicated sockets on theMUS 201. CGI scripts program 207 then initiates thetranslation/packet-forwarding server 208 within MUS 201 to join therequested Multicast group or groups (one group per selected media type)associated with the selected session. CGI scripts 207 that initiatedsuch join action to take place then adds that client's Unicast addressto a list of receivers for the requested Multicast group or groups.Server 208 then forwards Multicast packets received from the client tothe list of receivers for each joined group and translates the Multicastaddress of packets received from the joined group to the Unicast addressof the joining client.

The steps associated with a client joining a session are detailed withreference to the flowchart in FIG. 4. At step 401, the SDP listenerprocess accumulates IP Multicast network 101 directory announcementsinto database 204. The database is filtered and converted into astandard HTML page, where each URL points to information pertaining toeach Multicast session in the filtered database. At step 402, the HTTPserver 206 listens for requests for Multicast sessions/URLs on the HTMLpage which contains a list of URLs describing the sessions. At step 403,a request is received from a client for a copy of the page containingthe Multicast sessions. At step 404, the HTTP server 206 sends a messageback to the client requesting the to enter a login id and a password forauthentication purposes. If the user is successfully authenticated atstep 405, at step 406, server 206 returns a page to the clientcontaining a list of sessions. When, at step 407, the user of the clientbrowses the page and requests a session indicated by a URL, at step 408,server 206 returns a page containing details of the session and buttonsenabling the client to request the session to start, as well as otherfunctions to be described later herein. At step 409, the client starts aselected session (or specific media in the session) by “pressing” abutton on the HTML page. The server 206 then launches a control script207. At step 410, the control script 207 sends a response to the clientcontaining information about which multimedia tool to launch, and whatUDP sockets (the Unicast IP address on the MUS and associated ports) tolisten to and to send information to corresponding to the requestedgroup. It can be assumed that at least one socket is used. A secondsocket may be used to send control/status information from each memberof the Multicast group. At step 411, the same control script 207 ofserver 206 causes the Multicast-to-Unicast gateway to join therequested-for Multicast group and adds the requesting client to the listof receivers for the requested for Multicast group. For each requestingclient, server 206 also converts the address of the Multicast IP packetsof the requested-for group to the Unicast IP address of the requestingclient, and sends the Unicast IP packets to the requesting client usingthe well-known User Datagram Protocol (UDP). Server 206 also transmitsany packets it receives from the client to any other Unicast client onthat Multicast group, as well as to the Multicast group address itselfon the IP Multicast network 101. At step 412, when the client exits fromthe multimedia tool, server 206 detects that status messages from theclient are no longer being sent and removes the client from the list ofdestinations for the particular Multicast group.

The steps associated with a client creating a new session are detailedin FIG. 5. At step 501, as previously described in connection with thesteps in FIG. 4 associated with a client joining a session, the SDPlistener process 202 accumulates IP Multicast network 101 directoryannouncements into database 204. The database is filtered and convertedinto a standard HTML page, where each URL points to informationpertaining to each Multicast session in the filtered database. At step502, the HTTP server 206 listens for requests for Multicastsessions/URLs on the HTML page which contains a list of URLs describingthe sessions. When a user of a client requests a copy of the pagecontaining the Multicast sessions at step 503, the server 206, at step504, sends a message back to the client requesting a login id andpassword for authentication purposes. If authentication is successful atstep 505, at step 506, server 206 returns a page to the clientcontaining a list of sessions. When the user browses that page andrequests a specific URL, server 206, at step 507, returns a pagecontaining details of the session and buttons enabling the client torequest a session to start, to create a new session, to edit or deletean existing session, and to record the current session, the laterfunctionality to be described hereinafter. When, at step 508, the user“presses” a button to create a new session, at step 509, server 206returns a form to the client requesting the client to enter a login idand password appropriate for creating a new session, which may or maynot be the same as the login id and password associated with the initialauthentication process. If, at step 510, the client is authenticated forsession creation, server 206 returns at step 511 a form to the clientwith fields to be filled in by the user for the session name, sessiondescription, a URL address for more detailed or related information, afield to set the Multicast packet Time to Live value (TTL), selectionboxes for various media tools, and associated coding formats, Multicastaddresses and ports to use, the name and phone number of a responsibleperson, and the duration of the session announcement. When the userfills in the fields and selects a create button, the filled-in form isreturned to the server 206. Server 206 checks that certain key fieldsare correct and filled in. If not, an error message is returned to theclient; otherwise server 206 stores the data in a file that is indexedto the login id used to access the form. The data, at step 512, is thenmade available to a Session Announcement Protocol (SAP) process 210 onMUS 201. At step 513, the SAP process 210 periodically announces thesession onto a well-known Multicast IP address and port reserved forsuch announcements for a period of time equal to the duration field asentered in the form. When this time period expires, the SAP process 210ceases to announce the session.

A Unicast endpoint can also, in accordance with the present invention,request that a session be recorded for later rebroadcast or retrieval ondemand. To effect such recording, in FIG. 1, a recorder 125 is connectedvia LAN 126 to the IP Multicast network 101. The flowchart in FIG. 6details the steps associated with a user at a client terminal requestingthat a session be recorded. As described previously, the client must begiven a login id and password to enable access to the HTTP server 206which has a list of the IP Multicast sessions. At step 601, the SDPlistener process 202 accumulates IP Multicast network 101 directoryannouncements into sessions database 204. The database is filtered andconverted into a standard HTML page, where each URL points toinformation pertaining to each Multicast session in the filtereddatabase. At step 602, server 206 listens for requests for Multicastsessions/URLs on the HTML page which contains a list of URLs describingthe sessions. When, at step 603, a user requests a copy of the pagecontaining the Multicast sessions, at step 604, server 206 sends amessage back to the client requesting his or her login id and passwordfor authentication purposes. If, at step 605, authentication issuccessful, at step 606, server 206 returns a page to the clientcontaining a list of sessions. When a user browses that page andrequests a URL, server 206, at step 607, returns a page containingdetails of the session, and buttons enabling the client to request asession, to start the existing session, to create a new session, to editor delete an existing session, or to record the current session. At step608, the user “presses” a button to record a session and, at step 609,server 206 returns a form to the client requesting the user to enter alogin id and password for recording purposes, which may or may not bethe same as the login id and password used in a previous step. If, atstep 610, the client is authenticated for session recording, server 206returns a form to the client with fields to be filled in or options tobe selected. The form contains selection boxes for the various media ofthe session, and start and stop date and time fields, as well as a startbutton to “push” when the form is complete. When the user fills in thefields and selects the start form, the filled-in form is returned toserver 206. Server 206 checks that certain key fields are correct andfilled in. If not, an error message is returned to the client; otherwisethe server stores the data in a file that is indexed to the login idused to access the form. At step 611, server 206 sends a message to therecording server 125 to start the recording at the specified date andtime. The recording server 125, at step 612, then records the session atthe appropriate time using a well-known program for reading IP Multicastpackets in the RTP format. At step 613, the stored session is retrievedon-demand by the user.

To support dial-up Unicast endpoint users, prior art transcoding andrate adapting gateways, such as transcoder/adapter 127 in FIG. 1, aredeployed within IP Multicast network 101. As shown in FIG. 1,transcoder/rate adapter gateway 127 is connected via LAN 128 to the IPMulticast network 101. Alternatively, transcoder/rate adapter gateway127 may be co-resident with either MUS 120 or 121. The gateway mapsexisting high bandwidth sessions into low bandwidth versions of the samesession suitable for 28.8 kbps modems or ISDN adapters by using framediscarding algorithms. Announcements about these rate-controlledsessions are then placed in the database of IP Multicast network 101sessions.

An alternative mechanism for invoking the transcoding/rate adaptinggateways could be based on user/client action. The steps associated witha client requesting that a session be automatically transcoded/rateadapted are detailed in FIGS. 7A and 7B. As described previously, theuser of the client must be given a login id and a password to enableaccess to HTTP server 206. As described previously, at step 701, the SDPlistener process 202 accumulates IP Multicast network 101 directoryannouncements into database 204. The database is filtered and convertedinto a standard HTML page, where each URL points to informationpertaining to each Multicast session the filtered database. It isassumed that a required bandwidth parameter and maximum packet lossparameter are associated with each session. For sessions using a form aspreviously described, this is accomplished by the use of additionalfields corresponding to these parameters in the “new session” formdescribed hereinabove. At step 702, HTTP server 206 listens for requestsfor Multicast sessions/URLs on the HTML page containing a list of URLSdescribing the sessions. When, at step 703, a request is received from aclient for a copy of the page containing the Multicast sessions, at step704, server 206 sends a message back to the client requesting a login idand password for authentication purposes. If, at step 705,authentication is successful, at step 706, server 206 returns a page tothe client containing a list of sessions. When after the user browsesthe page, and at step 707, selects a session and group(s) within asession, server 206, at step 708, returns a page containing details ofthe session and buttons enabling the client to request a session tostart, to create a new session, to edit or delete an existing session,or to record the current session. At step 709, the user pushes a buttonto request the session or specific media in the session to start. Inresponse thereto, at step 710, server 206 launches a control scriptwhich sends a series of test packets to the client. At step 711, theclient echoes these same packets back to the server 206. At step 712,the server computes an estimate of the available bandwidth and packetloss along the path between the server and the client by comparing thetransmitted test packets with the received test packets. If, at decisionstep 713, the performance requirements of the session exceed that of thepath, at step 714, rate-adaptation or transcoding is needed. If not, theprocess continues as previously described for a client joining anexisting session. If rate adaptation or transcoding is needed, at step715, one out of plurality of lower rate encodings is selected based onthe measured characteristics of the path. It is to be assumed that afinite set of rate adapted sessions are allowed by the server 206. As anexample, if the original session was 128 kbps, rate adaptation to 56kbps, 28.8 kbps and 20 kbps may be allowed. At step 716, HTTP server 206sends a message to the transcoding/rate adapting gateway 127 indicatingwhich Multicast group is to be joined, to what bandwidth the media-typeshould be rate-adapted, what (if any) alternative coding needs toapplied, and what new Multicast address and port number the transcodedsession should use. At step 717, a determination is made whether thatgroup has already been rate-adapted/transcoded. If yes, at step 718, theMulticast address and port number corresponding to the alreadyrate-adapted/transcoded group is used and the process proceeds to step719. If no, at step 720, the transcoder/rate adapter gateway 127 isinvoked to transcode the session with appropriate bandwidth parametersand to re-Multicast the resulting lower-bandwidth session using adifferent Multicast address and port number. At step 719, the server 206sends a message to the client containing information about whichmultimedia tool to launch, and what UDP sockets (the Unicast IP addresson the MUS and associated ports) to listen to and to send information tocorresponding to the requested session. At step 721, the control-scriptcauses the MUS gateway to join the requested Multicast group and add therequesting client to the list of receivers for requested Multicastgroup. For each requesting client, the server converts the address ofthe Multicast IP packets of the requested-for group to the Unicast IPaddress of the requesting client, and sends the Unicast IP packets tothe requesting client using the well-known User Datagram Protocol. Theserver also transmits any packets it receives from the client to thelist of receivers for that Multicast group, as well as to the Multicastgroup address itself. At step 722, when the client exits the multimediatool, the HTTP server 206 detects that status messages from the clientare no longer being sent and removes the client from the list ofdestinations for the particular Multicast group.

The system described hereinabove is highly scaleable in that the MUS andother servers may be distributed throughout the Multicast-enabledportion of the network. If the number of such endpoints is likely to belarge, for efficiency, it is preferred that the servers be located closeto the Unicast client endpoints. For small conferencing group, thelocation or distribution of the MUS's is not important. Advantageously,the present invention enables any IP Multicast-based service to begradually introduced and rolled out on a small scale before all clientsare enabled for IP Multicast. As clients become IP Multicast enabled,there could be a transition to a hybrid IP Multicast mode of operationwhere some clients are IP Multicast connected and others are not.

Although described in connection with Unicast IP endpoints connecting toan IP Multicast network 101 such as the MBONE, the present inventioncould be employed with any IP Multicast network. Further the presentinvention could be employed with any Multicast network and Unicastnetwork that operate in a similar fashion to IP Multicast and Unicastnetworks.

The above-described embodiments are illustrative of the principles ofthe present invention. Other embodiments could be devised by thoseskilled in the art without departing from the spirit and scope of thepresent invention.

The invention claimed is:
 1. A method of providing access to a Multicastsession on a Multicast network to a Unicast client on a Unicast networkcomprising: accumulating at a gateway server interconnecting theMulticast network and the Unicast network directory information relatingto Multicast sessions on the Multicast network; supplying the Unicastclient with the directory information accumulated by the gateway server;receiving at the gateway server from the Unicast client a request tojoin a selected session chosen from the directory information; joiningthe requested-for Multicast session at the gateway server on behalf ofthe Unicast client; converting at the gateway server an address ofMulticast packets received on a Multicast address associated with therequested-for session to a Unicast address of the Unicast client andsending the Multicast packets received by the gateway server on theMulticast address to the Unicast client; and transmitting packetsreceived at the gateway server from the Unicast client to the Multicastaddress associated with the requested-for session.
 2. The method ofclaim 1 wherein the request to join a Multicast session is a request tojoin at least one group of a plurality of groups associated with theMulticast session, each of said plurality of groups having an associatedMulticast address.
 3. The method of claim 2 wherein each of said atleast one group is associated with a different media type.
 4. The methodof claim 1 further comprising authenticating the Unicast client beforesupplying the Unicast client with the directory information.
 5. Themethod of claim 4 wherein the authenticating comprises receiving at thegateway server a login identity and a password from the Unicast client.6. The method of claim 1 wherein the Unicast and Multicast networks areIP (Internet Protocol) Unicast and IP Multicast networks, respectively.7. The method of claim 6 wherein packets are transmitted between thegateway server and the Unicast client using the User Datagram Protocol.8. The method of claim 6 wherein the directory information comprises anHTML page having a plurality of URLs each pointing to informationassociated with a Multicast session on the IP Multicast network.
 9. Themethod of claim 6 wherein the IP Multicast network is the MBONE.
 10. AMulticast-Unicast server comprising: means for accumulating from aconnected-to Multicast network directory information relating toMulticast sessions on the Multicast network; means for receiving from aUnicast client on a Unicast network connected to the server a request tojoin a Multicast session from among the Multicast sessions accumulatedin the directory information; means for joining the requested-forMulticast session on behalf of the client; and means for converting theMulticast address of the Multicast packets on the requested-forMulticast session to a Unicast address associated with the Unicastclient and for sending the Multicast packets on the requested-forMulticast session to the Unicast client at that Unicast address, and forsending packets received from the Unicast client to the Multicastaddress associated with the selected session.
 11. The server of claim 10wherein the Multicast network and the Unicast network the server isconnected to are an IP (Internet Protocol) Multicast network and an IPUnicast network, respectively.
 12. The server of claim 11 whereinpackets are transmitted between the server and the Unicast client usingthe User Datagram Protocol.
 13. The server of claim 10 wherein therequest to join a Multicast session is a request to join at least onegroup of a plurality of groups associated with the Multicast session,each of plurality of groups having an associated Multicast address. 14.The server of claim 13 wherein each of said plurality of groups isassociated with a different media type.
 15. The server of claim 10wherein the means for converting also sends packets received from theUnicast client to another Unicast client associated with the server andwhich said another client is also connected to the session.